[NFC][Inliner] Introduce another multiplier for cost benefit analysis and make...

[NFC][Inliner] Introduce another multiplier for cost benefit analysis and make multipliers overriddable in TargetTransformInfo.

- The motivation is to expose tunable knobs to control the aggressiveness of inlines for different backend (e.g., machines with different icache size, and workload with different icache/itlb PMU counters). Tuning inline aggressiveness shows a small (~+0.3%) but stable improvement on workload/hardware that is more frontend bound.
- Both multipliers could be overridden from command line.

Reviewed By: kazu

Differential Revision: https://reviews.llvm.org/D153154